As is known in the art, a laser is a device that produces optical radiation using a population inversion in an active laser amplifying medium to provide light amplification by stimulated emission of radiation. The active medium is placed in an optical resonant cavity which provides positive feedback to generate oscillation. The resonant cavity is typically formed by surfaces or gratings which present a relatively high reflection to signals having a desired wavelength propagating within the cavity. The medium is conventionally pumped with intense energy from a pumping source to obtain the population inversion.
A fiber laser refers to a device in which the active laser amplifying medium is an optical fiber. Such a fiber typically has a core doped with an active element, for example, a rare earth element, and one or more cladding layers surrounding the core. When fiber lasers are used in practical communication systems, the output power of the laser is limited by the power available from the pump sources which pump the fiber.
A conventional technique of pumping the fiber core is to generate pumping power by an array of pump sources and launch the pumping power launching into a cladding layer using a technique commonly referred to as "cladding pumping". In a cladding pumped laser, a core is surrounded by a multi-mode cladding layer which, in turn, is surrounded by an outermost cladding layer. A relatively high-power multimode pumping signal from an array of sources is launched into the cladding. As the pumping energy propagates along the cladding layer, the energy is absorbed by the core, thereby pumping the core.
In order to insure that most of the pumping power is absorbed by the core, the optical fiber used in a fiber laser can have a length of 50 meters or more. A laser constructed with such a fiber has a very long laser cavity which can oscillate in many longitudinal modes. These modes have frequencies that are separated by an intermodal spacing value which is dependent on the round-trip propagation time of the light in the oscillator cavity.
One problem with multiple longitudinal modes in continuous wave lasers is the modes can beat together producing noise at the beat frequencies. Noise can also be produced by a phenomenon called "mode-locking." Although the longitudinal modes of a laser oscillator normally oscillate independently, their phases can be locked together by an external mechanism. For example, one such external mechanism could be absorption centers in the fiber in the region in which the light propagates. When the mode phases are locked together, the mode amplitudes combine to generate pulses in the output which appear as noise in a multiple longitudinal mode laser.
One mechanism for reducing mode beating and mode locking noise is to insure that the laser oscillates in only one longitudinal mode. An article entitled "Stable Single Mode Hybrid Laser with High Power and Narrow Bandwidth", P. A. Morton, V. Mizrahi, T. Tanbun-Ek, R. A. Logan, P. J. Lemaire, H. M. Presby, T. Erdogan, S. L. Woodward, J. E. Sipe, M. R. Phillips, A. M. Sergent and K. W. Wecht, Applied Physics Letters, v. 64, n. 20, May 16, 1994 discloses the use of a chirped fiber Bragg grating in a hybrid laser with a semiconductor--fiber cavity. The cavity is formed by coupling a semiconductor laser diode with an anti-reflection coating to an optical fiber. A chirped fiber Bragg grating forms one of the reflectors which define the laser cavity. Such a grating can be created using conventional doping techniques and can be arranged to transmit a portion of the laser light. Consequently, such gratings can be used as the output coupler of a laser cavity. The article describes that such a chirped Bragg grating can stabilize a single longitudinal mode when the chirped grating is oriented so that the grating period decreases in the direction towards the end of the laser cavity--the "blue" end of the grating is at the output coupling end of the cavity.
However, it would be desirable to decrease further the amount of noise in fiber laser output signals, especially in high-power lasers.